Ozone is a powerful oxidisation agent for organic substances and inorganic compounds containing elements with several oxidisation degrees. There are diverse areas of application for ozone, one of which is its use in water treatment.
Technically, ozone can be produced by passing a silent electrical discharge through gas containing oxygen. Silent electrical discharge is, in contrast to spark discharge, to be understood as a stable plasma discharge or corona discharge. Molecular oxygen is dissociated into atomic oxygen. The reactive oxygen atoms subsequently attach themselves to molecular oxygen in an exothermic reaction and form tri-atomic molecules, i.e. ozone. The ozone yield depends inter alia on the electric field strength and operating temperature.
Moreover, a dependence on gas composition has been observed. The dependence on operating temperature rests on the fact that ozone decomposes more rapidly again into molecular oxygen at higher temperatures and, due to the resulting displacement of the equilibrium between the originating and disintegrating ozone, the available ozone concentration is less.
Higher field strengths, which likewise lead to increased ozone yield, can be achieved inter alia by reducing the gap and by selecting dielectrics with higher relative dielectricity constants. Doped glass or ceramic materials are used for dielectrics with higher relative dielectricity constants. However, dielectrics made of ceramic materials have the disadvantage that they are non-homogenous and, in practice, may have lower dielectric breakdown strength than homogenous materials. Furthermore, high-grade ceramic materials in the form of moulds with high dimensional stability are extremely expensive and thinner dielectrics increase the risk of a dielectric breakdown.
Limits have been established for reducing the gap due to unpreventable manufacturing tolerances along with bending and buckling due to mechanical stresses and heat expansion in operation. Since an increase in field strength by reducing the gap width and by using dielectrics with high dielectricity constants leads to a significant increase in manufacturing costs, financial limits have been established here.
A device of the type referred to above is known from WO 93/16001. The electrically and thermally conductive, gas-permeable arrangement is formed from a number of helical coils, which form a series of curved surfaces, between which and the adjacent electrode an electrical corona discharge is formed. The previously known device is substantially cylindrically symmetrical. The arrangement together with a conductor resting inside it forms the inner electrode in all embodiments. Said conductor is a wire or tube and is arranged and centred mechanically on the tubular dielectric. The arrangement per se is a filler that has no centring task.
A device of the type referred to above is known from JP 1-51303, which is likewise substantially cylindrically symmetrical. Two tubes are held against each other centrally by peripheral spacers. The annular gap between said spacers is filled by the arrangement which is described as filler material. This is arranged in an irregular manner, which is inevitably the case with a filler material, and it does not have the task of mechanically centring the inner tube inside the outer tube.
In both cited documents, a metallic conductor is provided in the middle of the electrode arrangement as a support for the filler material, which also serves as an electrical contact with the filler material.
The closest prior art is disclosed in EP 0 789 666 B1. Said document describes ozone generators with a directly cooled external electrode and a rod-shaped metallic inner electrode with a dielectric resting between them. A knitted wire mesh is arranged between the electrodes and the dielectric, which improves firstly the thermal transfer from the feed gas to the cooled electrodes, and secondly, exhibits a plurality of cavities for silent discharge.